Low power high frequency discharge plasma generator



Dec. I 7, 1968 SEHCHI MURAYAMA 3,417,287

LOW POWER HIGH FREQUENCY DISCHARGE PLASMA GENERATOR Filed Sept. 28, 19662 Sheets-Sheet 1 2 PFi/OR ART F/G 6 PRIOR ART 1) A 4 A 6-5 .h

3\ A K 4 Y L INVENTOR 851mm MuRnYnnq ATTORNEY 1968 SEHCHI MURAYAMA3,417,287

LOW POWER HIGH FREQUENCY DISCHARGE PLASMA GENERATOR 2 Sheets-Sheet 2Filed Sept. 28, 1966 INVENTOR 850cm muaammn ATTORNEY United- StatesPaten 3,417,287 LOW POWER HIGH FREQUENCY DISCHARGE PLASMA GENERATORSeiichi Murayama, Hachioji-shi, Japan, assignor to Hitachi, Ltd., Tokyo,Japan, a corporation of "Japan Filed Sept. 28, 1966, Ser. No. 582,699Claims priority, application Japan, Oct. 8, 1965, 40/ 81,593 8 Claims.(Cl. 315-111) ABSTRACT OF I THE DISCLOSURE A high frequency dischargegenerator including a coaxial waveguide wherein the outer conductorthereof has an inner diameter which is less than 30 mm., the outerconduct-or of the coaxial waveguide extending beyond the end of theinner conductor so as to cover a high frequency plasma produced at thetop of the inner conductor, and

characterized in that carrier gas and a sample to be an- This inventionrelates to improvements in a high fre-' quency discharge apparatus andit is an object of the invention to stabilize discharge, to improveexcitation efficiency of a sample and to provide a high frequencydischarge apparatus having a structure convenientfor observation'of aflame.

Spectroscopic analysis of a sample based on the principle of highfrequency discharge has been conventionally performed by use of anapparatus as shown in FIG. 1. When microwave power is supplied from theleft of a coaxial waveguide 1, a strong high'frequency electric field isinduced at an electrode 4 placed at the top of the inner conductor of adischarge coaxial waveguide 2 so that a discharge plasma 6 appears atthe instant when a discharge gas is introduced from a discharge gasinlet 5.

In this case, a sample to be analyzed is dissolved into a solution,atomized with a sprayer and introduced together with the discharge gasfrom the discharge gas inlet 5. Since the sample thus introducedluminesces in the plasma flame 6, the sample may be analyzed byobserving the luminescence. In such a conventional apparatus, the sizeof the discharge coaxial waveguide 2 is adapted to that of an outputflange of a magnetron, which is a microwave generator, and so there hasbeen used, for instance, a high frequency discharge apparatus comprisingan outer conductor having an inner diameter of about 76.2 mm. and aninner conductor of outer diameter of about 33.3 mm. as shown in FIG. 1,or at least a rather large high frequency discharge apparatus comprisingan outer conductor of inner diameter of about 58.8 mm. and an innerconductor of outer diameter of about .16.9 mm. Accordingly, it hasbeen adefect of such a conventional appa ratus that a microwave source capableof producing rather large power above 1 kw.) is required. Forinstance,-in case whererare gas, such as argon, is used as the dischargegas and the power is low, the core of a flame swerves and becomesunstable as shown in FIG. 2. Moreover, the greater the power becomes,the more the electrode 4 will be exhausted to make discharge unstable.Since the electrode material will be mixed into the plasma flame, thespectra of the elements composing the electrode material will beobserved. The above facts constitute great drawbacks of such anexcitation source for tion, there is provided a high frequency dischargeapparatus composed so that'all the sample introduced may pass through aplasma flame of'a high temperature suificient tomake the sampleluminesce.

5 As is well known, only a charged particle, for example, an electron oran ion, can receive energy directly from a high frequency electricfield. Such charged particles receive energy from the electric field andtransfer it to neutral atoms by way of collision. In order for dischargeto continue, there must be a balance between the energy which a chargedparticle receives from an electric field and that which the chargedparticle gives to a neutral atom through collision. In high frequencydischarge'under ordinary atmospheric pressure (in the vicinity of thenormalstate), the energy an electron gains from an electric field ismuch larger than the energy received by an ion from an electric field.Therefore, it is sufficient to take only electrons into account forconsideration of energy balance. The energy W which an electron of massm and charge e gains from an electric field per unit time, is

. expressed in the form,

where E is an effective strength of a high frequency electric field and'y is a collision frequency. When 7,, is expressed'in terms of a meanfree path h and electron temperature T Equation 1 becomes,

v vsmucre' where k denotes a Boltzmann constant.

Now, as is known, the energy which an electron gives .35 to a neutralatom per single collision is equal to,

2 e M 'Ye Since W =W in an equilibrium state, there follows fromEquation 2,

Now, the temperature of a chemical reaction flame used for generalspectroscopic analysis is at most 3000 K. and it is known that it isimpossible to analyze a substance having a high excitation energy, forinstance zinc, by using such a flame of chemical reaction as an excita-1 ti'on source. Therefore, the temperature of 3000 K. can- 6 not besufiicient as an excitation source for analysis. On g the other hand,since, as is known, a substance like zinc may be easily excited in anarc discharge whereby the temperature of 5000 K. is available, it may besaid that the temperature of 5000 K. is suflicient for excitation of asample. However, a higher temperature is not always suitable foranalysis, for the higher the temperature rises, the more the backgroundof continuum will be included in the spectrum, and, consequently, theS/N (signal-tonoise) ratio becomes accordingly worse. When the spectralline intensity is measured with a wave length-scanning monochromator,the signal-to-noise ratio is defined as a ratio of the spectral lineintensity to the background con-- W 3 1 Jamar. 2

M As

tinuum intensity. Thus, it is concluded that the temperature of at least4000 K. is necessary. Now, by setting T =4000 K. in Equation 4, theeffective value of a high frequency electric field strength E will beobtained. Discharge gas most suitable for an excitation source foranalysis is one which has a small number of spectral lines constitutinga background and which is cheap and easy to obtain and so usually argongas is employed. In this case, a sample is introduced into the plasma inthe form of an aqueous solution and accordingly substances other thanargon, such as OH molecules, are present in the plasma, the partialpressure of which is considered to be about A of that of argon. Undersuch a condition, the collision with atoms like argon having a smallcollision cross section due to Ramsauer-Townsend effect may beneglected. Since at high temperature most of the water molecules areconsidered to be dissociated into OH molecules, R is estimated from thecollision cross section of an OH molecule to be 7\ 1.4 10 [m.] if thepartial pressure of OH molecules is assumed to be atm.

From this,

E=5.7 10 [volt/rn.] (5) As has become evident from the foregoingdiscussion, it is necessary to make all the samples pass through theplasma having an electric field stronger than that given by Equation 5.Thus, the electric field strength at the inner surface of an outerconductor where an electric field is Weakest must be larger than thevalue given by Equation 5. Since an electric field strength in a coaxialwaveguide depends only on the power supplied and the diameter of thecoaxial waveguide, the maximum diameter of the coaxial waveguide may bedetermined from the minimum condition of electric field strength givenby Equation 5 if the diameter of the coaxial waveguide and the electricpower are known. When the device is to be used as an excitation sourcefor spectroscopic analysis, other conditions, for instance stability ofdischarge, must be fulfilled. These conditions depend on a highfrequency. electric power. When the power is too small, the light runsshort of energy and the discharge becomes unstable. In the presentinvention, measurement has been made over a wide range of electric powerfrom a few tens of watts to a few hundreds of watts and the minimumusable electric power has turned out to be about 100 w. When arectangular waveguide is connected to a coaxial waveguide, thecharacteristic impedance of the coaxial waveguide is usually set to be5052. Under said condition, the maximum inner diameter of the outerconductor may be calculated as follows:

Effective voltage: (50X 100) ==7l [v.1 Etfective current: 100/50 1.4[amp.]

If the inner diameter of the outer conductor is set to be 2R, theelectric field strength E at the inner surface of the outer conductor isexpressed, from a well-known formula of coaxial tubes, in the form,

E=60 (effective current) 1/R [volt/m.] (6) From Equations 5 and 6,

R=1.5 1O [m.] (7) From this, the maximum value of the inner diameter ofthe outer conductor becomes,

Therefore, if the inner diameter of the outer conductor is made smallerthan 30 mm., the strength of the electric field becomes larger than thevalue of Equation 5 and the electron temperature T becomes higher than4000 K. under working conditions. Thus, it is secured that all thesamples pass through a plasma having a temperature higher than 4000 K.to produce light for spectroscopic analysis with a good excitationefficiency.

If a gas other than argon is used, the mean free path x, of a chargedparticle becomes much smaller than the value with argon andcorrespondingly the required electric field strength becomes larger thanthe value of Equation 5. Accordingly, the inner diameter of the outerconductor must be smaller than the value given by Equation 8.

For a better understanding of the present invention, reference will bemade to the following description of an embodiment of the invention,taken in conjunction with the accompanying drawings, in which;

FIG. 1 is a longitudinal sectional view of a conventional high frequencydischarge generator;

FIG. 2 shows the state of a discharge plasma flame generated by aconventional high frequency discharge generator;

FIG. 3 is a longitudinal sectional diagram of a high frequency dischargegenerator according to the present invention;

FIG. 4 shows the state of a discharge plasma flame generated by a highfrequency discharge generator according to the invention; and

FIG. 5 is a diagram showing spectral lines obtained. by spectroscopicanalysis with a high frequency discharge generator according to thepresent invention.

In an apparatus according to the present invention, microwave power issupplied from the left of a rectangular waveguide 7, shown in FIG. 3.Then a strong high. frequency electric field is produced at an electrode4 positioned at the top of an inner conductor 3 of a discharge coaxialwaveguide 2. When the discharge gas is introduced from a discharge gasinlet 5, a plasma flame 6 is generated. A sample for analysis isdissolved into a solution, atomized with a sprayer and introducedtogether with the discharge gas from the discharge gas inlet 5. Thesample thus introduced emits light in the plasma flame 6 and hence thesample may be analyzed by observing the luminescence. The inner diameterof the outer conductor of the coaxial waveguide must be less than 30 mm.and in FIG. 3 is shown an embodiment wherein the inner diameter of anouter conductor of a coaxial waveguide is 20 mm. Dielectric 8 serves toprevent leakage of the discharge gas into the rectangular waveguide 7and marks the end of the flow path of the discharge gas. The dischargegas inlet 5 and the dielectric 8 are separated by at least 10 mm. Thedistance between the discharge gas inlet 5 and the electrode 4 is madelarger than 20 mm. The discharge gas is introduced from the dischargegas inlet 5 in a direction tangential to the circumference of the outerconductor 2 of the coaxial Waveguide and is made to rise around theinner conductor 3 spirally. The outer conductor of the coaxial waveguideis extended to the extent that most of the flame is covered to a more orbarrier extent and holes or a slit 9 is provided thereon so that aniarbitary point on the flame axis may be observed. The electrode 4 atthe end of the inner conductor of the discharge coaxial waveguide isformed of aluminum.

The effects of each part of a discharge apparatus fabricated asdescribed above will be described in detail hereinbelow.

By making the inner diameter of the outer conductor of the dischargecoaxial Waveguide less than .30 mm, it becomes possible to cause thedischarge plasma flame 6 spread all over the interior of the outerconductor of the coaxial waveguide ias'shown in FIG. 3 even in case of alow electric power of -200 w. Since the outer conductor confines theflame in this case, a stable discharge as shown in FIG. 4 is obtained.Under such a low power, it is possible to generate a plasma flame 6which does not exhaust the electrode and which is ideal for anexcitation source for spectro chemical analysis. Also, when thedischarge plasma fiame 6 spreads over the interior of the outerconductor of the coaxial waveguide as shown in FIG. 3, all theintroduced sample enters the plasma flame to luminesce. According to aconventional method, however, much of the sample passes outside theplasma flame, and so the excitation efliciency was poor. The outerconductor extended so as to cover the discharge plasma flame 6 preventsthe sample from escaping from the plasma flame and accordingly it hasthe effect of facilitating the absorption of the microwave power in theplasma flame as well as adding to the excitation efliciency. Byextending the outer conductor, loss of microwave power due to leakagemay be reduced by several tens of decibels and hence it is possible toreduce the danger to the human body caused by microwaves as well as toimprove the efliciency of the generator. It has another eifect ofpreventing noise radiation resulting from discharge.

The discharge gas is introduced from the discharge gas inlet 5 into theinterior of the outer conductor in a direction tangential to thecircumference of the outer conductor and forms a spiral flow. In thiscase, unless the distance between the discharge gas inlet 5 and thedielectric barrier 8 forming the end of the flow path of the dischargegas is more than mm, the spray introduced from the discharge gas inlet 5turns to water drops on the dielectric '8 to cause loss of a sample.Also, unless the distance from the discharge gas inlet 5 to theelectrode 4 ,is more than 20 mm., a uniform spiral flow may not beobtained and the discharge becomes unstable.

The luminous state of a sample in the discharge plasma 6 dependsappreciably on the position in the plasma flame. For a sample to emitlight, three processes, is. (1) evaporation of a spray, (2) dissociationof molecules composing a sample material, (3) excitation of thedissociated element are required in advance. Since the ease with whichthese processes occur depends on the kind of a sample, the point in theflame where a maximum S/N ratio is obtained in the observation of thespectral lines of sample elements differ from sample element to sampleelement. Generally, it often happens that the central point of theflame, more than mm. above the end of the electrode, constitutes anoptimum observation point. In the present apparatus, holes or a slit 9is provided so that an arbitary point on the axis of the flame may beobserved and the apparatus is composed so that a position where amaximum S/N ratio is obtained for any element may be observed.

FIG. 5 shows an experimental result obtained with the present apparatuswhen an aqueous solution of zinc of 5 p.p.m. (parts per million) (ZnSOaqueous solution) is atomized with a sprayer and introduced withdischarge gas (argon) into a plasma flame. The light from the plasmaflame is lead into a monochromator and the output therefrom is receivedby a photomultiplier tube The current running\ through thephotomultiplier tube is recorded in a recorder while scanning thewavelength of the monochromator. It is concluded from' this result thatthe spectral line of Zn 4810 A. may be distinguished from backgroundnoises even when the concentration of zinc solution is much lower than 5p.p.m.

If the minimum detectable concentration is defined as a concentrationwhich can emit the spectral line twice as intense as background noises,it is concluded that the minimum detectable concentration of the presentapparatus is 0.3 p.p.m. :for zinc. This sensitivity is much better thanthat of the conventional apparatus, whose minimum detectableconcentration or zinc is 5 p.p.m.

Iclaim:

1. A high frequency discharge generator comprising:

plasma generating means for generating a discharge plasma flame in theform of a coaxial waveguide including inner and outer conductors,

power supply means for applying high frequency elec= trical energy tosaid coaxial waveguide, and

gas supply means for supplying a gas between the in ner conductor andthe outer conductor of said coaxial waveguide,

said outerfconductor having an inner diameter of less than 30 mm. andextending beyond said inner conductor at the end thereof at which theplasma flame is generated, so that a stable flame is generated at lowpower.

2. A high'frequency discharge generator as defined in claim 1 furtherincluding a dielectric barrier in said coaxial waveguide between saidgas supply means and said power supply means preventing passage of saidgas.

3. A high frequency discharge generator as defined in claim 2 whereinsaid gas supply means includes an inlet aperture in said outer conductorpositioned at least 10 mm. from said dielectric barrier.

4. A high frequency discharge generator'as defined in claim 3 whereinsaid inlet aperture is positioned more than 20 mm. from said one end ofinner conductor.

5. A high frequency discharge generator as defined in claim 2 whereinsaid gas supply means introduces gas intov said coaxial waveguide in atangential direction to the inner circumference of said outer conductorso as to produce a spiral rising of said gas around said inner con:ductor.

6. A high frequency discharge generator as defined .in claim 1 whereinsaid one end of said'inner conductor is provided with an electrode tipmade from aluminum.

7. A high'frequency discharge generator as defined in claim 6 whereinthe gas supplied by said gas supply means is argon.

8. A highfrequency discharge generator as defined in claim 7 wherein theportion of said outer conductor ex. tending beyond said inner conductoris provided with a viewing aperture, said power supply means introducingelectrical energy to said waveguide at the other end thereof.

References Cited UNITED STATES PATENTS 3,280,364 10/1966 Sugawara313-231 X 3,353,060 11/1967 Ya'ma-moto et a1. a..." 315 111 FOREIGNPATENTS 1/1963 France.

U.S. Cl. X.R.

